Stock option plan and stock-based compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
Jun. 30, 2018	Sep. 30, 2017
Number of Options
Beginning balance (in shares)	45,817,252
Granted (in shares)	7,676,414
Exercised (in shares)	(1,735,750)
Forfeited (in shares)	(3,387,668)
Ending balance (in shares)	48,370,248	45,817,252
Weighted Average Exercise Price
Beginning balance (in USD per share)	$ 9.20
Granted (in USD per share)	15.09
Exercised (in USD per share)	3.31
Forfeited (in USD per share)	13.05
Ending balance (in USD per share)	$ 10.08	$ 9.20
Additional Information
Weighted Average Remaining Contractual Term	6 years 8 months 12 days	6 years 10 months 24 days
Weighted Average Intrinsic Value	$ 243,060	$ 199,663
Options exercisable	30,582,952
Option exercisable - Weighted Average Exercise Price (in USD per share)	$ 7.70
Option exercisable - Weighted Average Remaining Contractual Term	5 years 4 months 24 days
Options exercisable - Weighted Average Intrinsic Value	$ 226,453
Options vested and expected to vest	45,413,361
Options vested and expected to vest - Weighted Average Exercise Price (in USD per share)	$ 9.80
Options vested and expected to vest - Weighted Average Remaining Contractual Term	6 years 7 months 6 days
Options vested and expected to vest - Weighted Average Intrinsic Value	$ 240,964

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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Number of options outstanding, including both vested and non-vested options.

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Number of fully vested and expected to vest options outstanding that can be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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